1. Field of the Invention
The present invention relates to a thermoplastic multi-walled pipes and a process for producing the same, and more particularly to thermoplastic multi-walled pipes each composed of two or more tubular members different in inside diameter concentrically arranged with a hollow portion therebetween.
2. Description of the Prior Art
It has been well known for many years that the thermoplastic multi-walled pipe composed of two or more tubular members different in inside diameter concentrically disposed with a hollow portion therebetween is excellent in heat insulation property because of its hollow portion or portions. However, such multi-walled pipes have hardly been brought into practical use, because because it has been difficult to join them to the conventional single wall pipe by such as sockets, elbows, Tee joints, etc.